This invention relates to impact ink transfer printing, and, in particular, to transparent sheets capable of receiving inks from impact ink transfer printers.
A familiar example of impact ink transfer printing involves the common typewriter. As is well-known, ink is carried on a fabric ribbon which, when struck against the paper by the character to be typed, transfers a certain amount of ink to the paper, thereby forming an image of the striking character. Because the ribbon is exposed to air for long periods of time, the ink must be of a type that will not dry to such an extent that it will be incapable of transfer. At the same time, the image formed on a paper surface must be permanent, i.e. the ink must dry on the paper so that it will not be easily smeared. These seemingly incompatible requirements, namely, non-drying on the ribbon and immediate drying on the paper surface, are reconciled by the capillary action of the paper. When the ribbon contacts the paper, the ink rapidly diffuses into the latter, providing a relatively permanent image thereon and leaving the surface thereof relatively dry. The amount of ink necessary to form the image is sufficiently small to allow ink transfer to occur without paper being saturated.
Although the fabric ribbons described above have, in recent years, been replaced in conventional typewriting by wax on film, or "carbon", ribbons, which give an image having higher resolution and optical density, the fabric ribbon has become standard for dot-matrix computer printers. These printers combine speed, low cost, and the ability to print graphic images as well as text. This graphics capability renders the dot-matrix printer desirable for preparing overhead transparencies. Overhead transparencies are frequently used in visual presentations wherein images are commonly projected with magnification onto a screen for simultaneous viewing by a plurality of observers.
The dot-matrix printer comprises a matrix of fine wires or pins mounted in a traveling head. Each pin is electromechanically activated, through the driver circuitry, typically by a computer, to move toward the ribbon and impact it at high speed, thereby pressing it against the paper receptor sheet to form a dot. By activating the appropriate pins at the appropriate times, a matrix of dots can be produced in the form of the desired character. Since the character is formed electronically by the activation of individual pins, the output is not limited to any one set of characters, and the printer can print characters which are not a part of any font, but simply parts of a larger graphic image. This feature would allow the dot-matrix printer to be a useful tool for making images on transparent polymeric film if the ink could be made to deposit and dry on non-porous, polymeric film, as opposed to porous paper sheet.
The problem of depositing and drying ink on transparent polymeric film occurs with many computer output devices other than impact printers. For example, pen plotters and ink-jet printers produce images on polymeric film by means of aqueous-based inks. An ink-receptive polymeric film for aqueous-based inks has been prepared by providing on the surface of a polymeric backing a transparent hydrophilic coating which absorbs the ink and optionally allows subsequent evaporation of the ink vehicle. Such absorbent coatings tend to have numerous undesirable features, such as lack of durability, stickiness, susceptibility to fingerprinting, and sensitivity to moisture. U.S. Pat. No. 4,301,195 discloses an ink-receptive polymeric film having a protective coating provided over the ink-absorbing layer, said protective coating providing the desired handling and durability properties, while still allowing a suitable rate of ink passage through to the absorbent layer. Japanese patent application No. 108541 (May 30, 1984) discloses protective layers which are porous and Japanese patent application No. 155442 (July 27, 1984) discloses protective layers which have microcracks.
Ink used with impact ink transfer printers contain liquid vehicles and colorants which differ considerably from those used with pen plotters or ink-jet printers. A common feature of vehicles for inks for pen plotters and ink-jet printers is the use of water, ethylene glycol monoethyl ether, or other low volatility solvents which are either highly polar or highly hydrogen bonded, or both. Because inks for ink-jet printers must exhibit electrical conductivity, they generally utilize water in their formulation. Further, because inks for pen plotters and ink-jet printers must pass through small openings in applicators, solid pigments which might clog such openings cannot be used.
The primary requirements for impact transfer printing inks are that they must provide images of acceptable density, while being present on the fabric ribbon in relatively low quantities. If the ink is present in too great of a quantity, the ribbon will become wet or sticky and will soil the printed sheet in areas where imaging is not intended. Accordingly, the impact transfer printing ink must be formulated to have a very high color strength. A secondary requirement of impact transfer printing inks is that once an area of the ribbon has impacted the receptor surface, that area must have its ink supply replenished by having ink flow thereto from the surrounding unused area. Efforts by ribbon manufacturers to produce useable ribbons within these constraints have resulted in a variety of ink formulations, but a common feature of most of them is the use of mineral oil and oleic acid as components of the liquid vehicle, and the use of carbon black or other solid pigments to achieve the requisite color strength. Accordingly, ink-receptive surfaces suitable for use with these inks must be oleophilic rather than hydrophilic, and cannot depend exclusively upon diffusion of vehicle and dissolved dye, but also require means of anchoring solid pigment thereto.
In addition to absorbing ink, the ink-receptive surface of the transparency film must also have the durability and freedom from stickiness required for reliable printing and handling.